A varistor has resistance that is changed based on a change in electric field. The varistor mainly serves to protect a semiconductor element from an abrupt change in voltage.
As electronic products are minimized and light-weighted with multi-functions, semi-conductor elements that are mounted on the electronic products are also highly integrated. Accordingly, varistors for preventing the semiconductor elements from mal-functioning or breaking are also highly integrated.
As highly integrated semiconductor thin film elements with various functions are developed and used, thin film type varistors that can be mounted together with the highly integrated semiconductor thin film elements are also required to be developed.
However, it is nearly impossible to manufacture a thin film type varistor through an existing procedure of manufacturing a thick film type varistor or a bulk type varistor.
This is because a temperature at which the thick film type varistor or bulk type varistor is manufactured is too high. In general, in the existing procedure of manufacturing the thick film type varistor or bulk type varistor, a sintering temperature of 1000° C. to 1300° C. is required. However, an electrode of a thin film type varistor and a semiconductor thin film which are vulnerable to heat are broken or damaged at the aforementioned temperature.
This is also because the varistor has to contain about ten types of additives (for example, bismuth oxide (Bi2O3), praseodymium oxide (Pr6O11), cobalt oxide (CoO), chromium oxide (Cr2O3), mangan oxide (MnO), erbium oxide (Er2O3), and the like) so as to have a good characteristic of a thin film type varistor. However, it is very difficult to deposit a thin film of the varistor by accurately adjusting the additives in an existing method of depositing a thin film.
A currently commercialized zinc oxide based varistor is a thick film type varistor or bulk type varistor with a non-linearity coefficient equal to or greater than 50. However, when the thin film of the varistor is deposited through an existing thin film deposition method such as a sputtering deposition method, a chemical vapor deposition method (CVD), and the like, the non-linearity coefficient is rapidly decreased to a value of 3 to 22.
In order to solve this problem a bismuth based varistor to which bismuth oxide is added is suggested. However, volatility of bismuth oxide becomes very high due to a high temperature sintering process when manufacturing the bismuth based varistor. Accordingly, reproducibility of the varistor is low. The bismuth based varistor easily reacts on other oxides to produce various electrically unnecessary phases therein, thereby deteriorating characteristics thereof.